while this is true, most A/M parts do decrease in cost relative to model year depreciation as well. Here you have an expensive mod that is 1/3 value of the car in a used market.

Yes, but it's a horses for courses kind of thing. Not all modifications are done because they are financially advisable... If you love a car's styling, then putting money into it to make it competitive with more modern equivalents (or your buddies turbo E36...) then more power to you! There is an emotional part to modding... Besides, if guys didn't buy parts and then part them out, the classified section would fall apart!